Experimental Results and Discussion

6.1.2.3.2Chemical / Elemental Specifications

6.1.3 Preliminary Rheological Tests

6.1.3.3 Experimental Results and Discussion

The results discussed in this section are for ferro silicon and magnetite #1 suspensions only.

Ferro silicon suspensions are used in the heavy medium separations of ores having a specific gravity in the range of approximately 2.5 to 4.0 [Collins et al. (1974)]. It is important to know the behaviour of these suspensions at different specific gravities, as this will influence the separation efficiency in the vessels. These tests were carried out to investigate the flow behaviour of the suspensions by measuring their shear stress versus shear rate in the rotational viscometer.

Ferrosilicon measurements were carried out at range of specific gravities between 2.0 and 3.0.

Below a specific gravity of 2.0 the suspensions became unstable, making measurement very difficult. Above a specific gravity of 3.0, the viscosity measurements also became difficult due to the much higher solids volume concentration, which resulted in clogging of the annular region between the bob and cup. An interesting observation was the fluctuation and irreproducibility of the results at the low and high specific gravities. At high specific gravities lower than expected viscosity readings were observed. This is due to the settling out of suspension of the media particles. This results in a supernatant in the region around the measuring bob. The viscosity of the suspension then becomes lower because of reduction in the solid content in that region.

Sometimes, if the settling rate of the solid particles is too high, the solids accumulate at the bottom of the cup and result in some resistance to the rotation of the bob. This has the effect of simulating high viscosities, resulting in inaccurate results. This was one of the motivations of adding a chamfer at the base of the rheometer cup.

Magnetite measurements were made at specific gravities ranging from 1.5 to 2.6. Above 2.6 the viscosity measurement became difficult because of the high viscosities associated with magnetite suspensions in this region. Because of this, magnetite suspensions are seldom used at specific gravities above 2.5 [Wills (1997)]. The stability of magnetite suspensions at lower specific gravities is much higher than that of ferrosilicon. This is due to its much lower density, which results in a higher solids volume percentage at similar specific gravities [Aplan et al. (1964)]. At specific gravities in excess of 2.6, viscosity measurement became difficult due to the same reasons as those for ferrosilicon.

CHAPTER 6 EXPERIMENTAL

Table 6.3 shows the solids volume percentage for ferrosilicon and magnetite #1 at the measured specific gravities.

Table 6.3 Ferrosilicon and Magnetite #1 solids volume percentage

<I> (%)

Specific gravity Ferrosilicon Magnetite #1

1.5

-

^13.7

1.6

-

^16.5

1.7 - 19.2

1.8

-

^22.0

1.9 - 24.7

2.0 17.4 27.5

2.1 19.1 30.2

2.2 20.8 33.0

2.3 22.6 35.7

2.4 24.3 38.5

2.5 26.0 41.2

2.6 27.8 44.0

2.7 29.5 -

2.8 31.5

-

2.9 33.0

-

3.0 34.7

-

Figure 6.10 and Figure 6.11 show the rheograms for the ferrosilicon suspensions at the specific gravities given in Table 6.3. Figure 6.12 shows the effect of specific gravity on the shear stress of the suspensions. These results were plotted by taking the shear stress of each of the suspensions at a shear rate of 1200 S-I. The results illustrate that there is an increase in the viscosity of the suspensions with specific gravity. This is due to an increase in the solids volume percentage.

Figure 6.13 and Figure 6.14 show the rheograms for the magnetite #1 media. Figure 6.15 shows the effect of specific gravity on magnetite #1 suspensions at a constant shear rate equal to 1000 s-

I. The rheograms for both mediums were plotted on two separate graphs for the sake of clarity.

CHAPTER 6

30.00 25.00

.r '5 20.00

~

rI}

rI} ~ 15.00

-

^rI}

_-

~ 10.00 .c 00.

,-,

N

5.00 0.00

60.00 50.00

~

40.00

rI}

~ 30.00

-

^rI}

_-

~ 20.00 .c 00.

10.00 0.00

o

o

EXPERIMENTAL

- .- sp.gr.2.0 -sp.gr.2.1 - - sp.gr.2.2

- +< - sp.gr.2.3 - - sp.gr.2.4 -sp.gr.2.5

200 400 600 800 1000 1200 1400 1600 1800 2000 Shear rate (S-I)

Figure 6.10 Ferrosilicon rheograms (sp.gr. 2.0-2.5)

- - sp.gr.2.6 - - sp.gr.2.7

- e- sp.gr.2.8 - - sp.gr.2.9 -sp.gr.3.0

,..¥.

.,;

. ; /

...

/ ' ..

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J .11 ~ •

.,;. ~II ^{' • .,; • -}

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~

-.«

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pi-....

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.

-'-

200 400 600 800 1000 1200 1400 1600 1800 2000 Shear rate (S-I)

Figure 6.11 Ferrosilicon rheograms (sp.gr. 2.6-3.0)

CHAPTER 6

35.00 30.00

-

";'5 25.00

~

(I) 20.00

(I)

~ 100

t; 15.00

100 ~

.: 10.00 _rr.; ~

5.00 0.00

EXPERIMENTAL

1.9 2.0 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3.0 3.1 Specific Gravity

Figure 6.12 Shear stress versus specific gravity for ferrosilicon suspensions at a shear rate of 1200 S·1

40.00 35.00

",-30.00 '5 Z 25.00

' - ' CIl CIl

~ 20.00

...

(I)

; 15.00 .: ~

00 10.00 5.00 0.00

o

- s p.gr.1.5

~sp.gr.1.6

- " sp.gr.1.7

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,.;w'.:"'"

~:A'" ___ ..--

". ~!.:::.--

200 400 600 800 1000 1200 1400 1600 1800 2000 Shear rate (S·l)

Figure 6.13 Magnetite #1 rheograms (sp.gr. 1.5-2.1)

CHAPTER 6

160.00 140.00

---

^120.00

~

e z

_{' - '} ^100.00

~

~ 80.00

Q,j

..

-

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..

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00 40.00 20.00 0.00

140.00 120.00

---

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80.00

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_~ ^60.00

-=

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00 40.00 20.00 0.00 0

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200

--

_ . - __ r-

----

EXPERIMENTAL

- " ' " _ &

...--

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.~-.--.-.-

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400 600 800 1000 1200 1400 1600 1800 2000

Shear rate (S·l)

Figure 6.14 Magnetite #1 rheograms (sp.gr. 2.2-2.6)

1.4 1.5 1.6 1.7 1.8 1.9 2 2.1 2.2 2.3 2.4 2.5 2.6 2.7 Specific Gravity

Figure 6.15 Shear stress versus specific gravity for magnetite #1 suspensions at a shear rate of 1000 s·

I

CHAPTER 6 EXPERIMENTAL

The ferrosilicon results show that the behaviour of the ferrosilicon suspensions ranges from Newtonian behaviour at low specific gravities, to dilatant behaviour at higher specific gravities.

Figure 6.10 shows that at a specific gravity equal to 2.0, the ferrosilicon medium is Newtonian.

There is an increase in viscosity with an increase in specific gravity. This is expected, since an increase in medium specific gravity results in an increase in the solids volume percentage, as illustrated in Table 6.3. The degree of dilatant behaviour appears to be increasing with specific gravity. Figure 6.11 also shows an increase in the dilatant behaviour of the medium as the specific gravity increases. Wills (1997), states that at a solid volume percentage above 30 %, heavy medium suspensions begin to exhibit non-Newtonian behaviour. This is evidenced from the results. In this case however, the onset of non-Newtonian behaviour begins at a specific gravity around 2.5, which has a solid volume percentage of 26.0 %. The onset of non-Newtonian behaviour at these specific gravities is due in part to the instability of the medium at low specific gravities. The solid particles settle out of the medium and accumulate at the base of the cup, slightly hindering the rotation of the measuring bob [Klein et al. (1990)]. This falsifies the results by giving the impression that a higher viscous medium is being tested. It will be shown later that measurement at these values begins to improve through the addition of slime, which acts as a stabilising agent.

The dilatant behaviour observed for the ferrosilicon suspensions is similar to that observed by Collins et al. (1976) and Ferrara et al. (1986). Both these authors found that milled ferrosilicon suspensions behave as pseudo-Newtonian fluids at low shear rates and specific gravities, and become dilatant at higher specific gravities. Collins et al. (1983) also showed that at specific gravities below 2.8 the behaviour of ferrosilicon suspensions is Newtonian, becoming dilatant at much higher specific gravities.

Figure 6.13 and Figure 6.14 show the shear stress versus shear rate relationship for the magnetite

#1 media. At specific gravities ranging from 1.5 to 2.0, the behaviour of the media is pseudo- Newtonian. At these specific gravities, the solid volume percentages are below 30 %. For specific gravities in excess of 2.0, the pseudo-plastic behaviour of the media increases. As shown in Figure 6.14, the viscosity rises sharply from a specific gravity of 2.5 to 2.6, the latter being the maximum specific gravity which could be handled by the viscometer. Although measurements were not made at zero shear rates, the graphs seem to suggest the existence of a yield stress with an increase in specific gravity. These results are similar to those observed by Lapsin et al. (1988).

CHAPTER 6 EXPERIMENTAL

The viscosities of the magnetite suspensions appear to be greater than ferrosilicon viscosities at the same specific gravity. However, this is not the most realistic way of comparing the viscosities, since the solid volume percentages are different for these media at the same specific gravity. A much better method is to compare the viscosities of the two media at the same solids volume concentration. Figure 6.16 and Figure 6.17 compare the rheograms of magnetite and ferrosilicon media at a solids volume percentage of 24 % and 33 %, respectively. At the solid volume percentage of 24 % the specific gravity of magnetite is 1.9, and the ferrosilicon specific gravity is 2.4. For 33 % solids volume the specific gravities are 2.2 and 2.9 for magnetite and ferrosilicon, respectively.

30.00 25.00

-

....

~

20.00

' - ' C"I.l C"I.l

~ 15.00

-

^C"I.l

_..

~ 10.00 .:: rJl

5.00 0.00

o

o F errosilicon

• Magnetite #1 -Magnetite #1 - - F errosilicon

200 400 600 800 1000 1200 1400 1600 1800 2000 Shear rate (S-I)

Figure 6.16 Shear stress vs shear rate at 24% volume solids

CHAPTER 6

50.00 45.00 40.00 ..-..

N

's

^35.00

Z ,-,30.00

'"

'"

f:

25.00

....

'"

... 20.00

~ ~ 15.00

rrJ

10.00 5.00 0.00

0

o F errosilicon

• Magnetite #1 -Magnetite #1 - - Ferrosilicon

200 400 600

EXPERIMENTAL

800 1000 1200 1400 1600 1800 2000

Shear rate (S-l)

Figure 6.17 Shear stress vs shear rate at 33% volume solids

Both of the above graphs show that at equal solid volume concentrations the magnetite media have a much higher viscosity than ferrosilicon media. This is one of the reasons why ferrosilicon media are used at higher densities, irrespective of the fact that they cost more than magnetite media. The higher viscosities of magnetite media are associated with the higher fines content.

This was also illustrated in the size distribution curves in Figure 6.1.

The preliminary rheological tests showed what type of behaviour magnetite and ferrosilicon suspensions followed. The results observed were similar to those found in literature. One of the biggest industrial challenges is the use of these suspensions at their critical specific gravities. For industrial separations using ferro silicon suspensions, the presence of slimes presents a problem by increasing the acquired viscosity of the suspensions. For magnetite suspensions the challenge is working mediums of this type at specific gravities greater than the conventional ones. It was thus required to look at the different methods available for viscosity reductions in heavy medium suspensions. The two methods chosen for this project are the use of a surface active agent, and the use of coarser media size distributions. These form the basis of Experiment series A, Band C.

In some industrial heavy medium separations mixtures of ferrosilicon and magnetite are used to make up the suspensions. The amount of ferrosilicon and magnetite used depends on the

CHAPTER 6 EXPERIMENTAL

separating vessel, as well as the ore being separated. As mentioned above, magnetite media have high fines content, thus mixtures used should not result in viscosities that affect the separation efficiency. However, the presence of moderate amounts of magnetite improves the stability of ferrosilicon suspensions.

Collins et al. (1983) did some experiments using, ferro silicon/magnetite suspensions, to evaluate the separation efficiency in dense media cyclones. Viscosity measurement was undertaken with cyclone 60 ferrosilicon and various mixtures of ferrosilicon with magnetite. Measurements were undertaken with a concentric cylinder rotational viscometer, at a temperature of 20 DC. They found that mixtures of ferrosilicon and magnetite were essentially pseudo-plastic at low magnetite levels (025% magnetite), but showed Bingham plastic conditions at higher magnetite levels (0 50% magnetite), yield stress increasing with the magnetite content of the medium. Their results indicated the large increase in viscosity caused by the use of magnetite with ferrosilicon and the problem of using magnetite at high separation densities and low shear rates.

Figure 6.l8 shows the effect of specific gravity on the viscosity of FeSilMag #1 suspensions at ratios 1: 1, 2: 1, and 1 :2. These graphs were plotted by taking the viscosity of each of the suspenSIOns at a shear rate equal to 1200 S·l. The results show that the viscosity of the suspensions increases with specific gravity. This effect becomes more pronounced at high specific gravities. The viscosities of the suspensions also increase with the amount of magnetite

#1 present in the suspension. This is due mainly to the amount fines introduced with the addition of magnetite #1.

Figure 6.19 to Figure 6.25 show the effect of shear rate on viscosity of 270D ferrosilicon- magnetite #1 mixtures at specific gravities 2.2-2.8, for FeSilMag #1 ratios 1: 1, 2: 1, and 1 :2.

CHAPTER 6

0.060 0.050

~ 0.040

~ ~ ' - '

C

^0.030

.•

r/)

o CJ

>

r/) ^0.020

0.010 0.000

1.9

-.- 1:1

" 0" 2:1 - 1:2

2.0 2.1

EXPERIMENTAL

,,. ... -

"'.... _(3 •

.... :::-::: :-.-:-.-:-.--: ~;.~ ^{........ e·····}

2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9

Specific Gravity

Figure 6.18 Effect of specific gravity on the viscosity of FeSi/Mag #1 suspensions at a shear rate of 1200 s'!

0.020 0.018 0.016

~ 0.014

....

~

^0.012

' - '

>. 0.010

...

.

^~

o 0.008

CJ

>

r/) ^0.006

0.004 0.002 0.000

- - 1:1 - • - 2: 1 - 1 : 2 - - FeSi

o

200

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..•. ---

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---._- ^{... -} -- ^- -- ^{. ..---} .-

400 600 800 1000 1200 1400

Shear rate (S·l)

.-' -- ^...

. -

1600 1800 2000

Figure 6.19 Effect of shear rate on viscosity for 270D ferrosilicon-magnetite #1 mixtures (sp.gr.2.2)

CHAPTER 6

0.025

0.020 ....

g

^0.015

-

~

...

~ 0.010

C.I

<I}

;>

0.005

0.000

- - 1:1 - • - 2: 1 - 1:2 - - FeSi

o

²⁰⁰

EXPERIMENTAL

.... _... --

^.-

^--....- ...

^{.. .. ..}

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-- ^-

^.,.

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- ^. -- ^- -- ^{- -- .} - ^{. -}

400 600 800 1000

-...- ...-.-- . ----

1200 1400 1600 1800 2000 Shear rate (S-l)

Figure 6.20 Effect of shear rate on viscosity for 270D ferrosilicon-magnetite #1 mixtures (sp.gr.2.3)

0.035

- - 1:1 0.030

~ 0.025 ....

's

Z 0.020

-.- 2:1 - 1 : 2

- - FeSi

~ .... ^{, "} ..

.... _'--- --- _~

-- -

^>.

.;; 0.015 c C.I

. ^- _{.. - -.. ---} .. ^{- --} _. ^{.. - --} . ^{- -} . ^-- . _{- -- .-.--} . ^{- - ... ---} ..

<I}

;>

0.010

--- ---

0.005 0.000

o

200 400 600 800 1000 1200 1400 1600 1800 2000 Shear rate (S-l)

Figure 6.21 Effect of shear rate on viscosity for 270D ferro silicon-magnetite #1 mixtures (sp.gr.2.4)

CHAPTER 6

0.045

- - 1:1 0.040

-.- 2:1 0.035 - 1 ; 2

-

^~

~. 0.030 - ... FeSi

~ 5 0.025

>.

;; 0.020

Q Cj

0.015

..

~

;;>.

0.010 0.005 0.000

0 200

...

••

...

' ... ,

EXPERIMENTAL

' ... -- -:-:-:'-:~-:-::-~---"""'--='"'II-=:"-:-

^·1

... ^-- -. ^- -. ^{- - .. -- -} ... ...•.. - .... . .. ^- ...

.. . ^{- . .... _} .•. ^_ . • . ^{_ . .... . _.- ... . _ .... _._} .. ^{_ ._ .... - .... _ ... _.-.}

400 600 800 1000 1200 1400 1600 1800 2000 Shear rate (S·l)

Figure 6.22 Effect of shear rate on viscosity for 270D ferrosilicon-magnetite mixtures #1 (sp.gr.2.5)

0.070

- - 1:1 0.060 - •• 2: 1

- 1 : 2

~ 0.050

N

'5 Z 0.040

'-"

>.

-

'r;; 0.030

Q Cj

~

;; 0.020

0.010 0.000

- - FeSi

o

200

... ,

...

-... - ... - -+- - ... - + - - ....

- - + - - + - - ... - .

. ^. .. ^{. .} ... ^- ... ^- .. ^{- - .} . ^{-- -} . ^- ... .

- ^{. - - -- .- - - .} -- ^{- - .} -.---.---.

400 600 800 1000 1200 1400 1600 1800 2000 Shear rate (S·l)

Figure 6.23 Effect ofshear rate on viscosity for 270D ferrosilicon-magnetite #1 mixtures (sp.gr.2.6)

CHAPTER 6

0.090

- - 1:1 0.080

- • - 2: 1 0.070 - 1 : 2

--

^..",

~. 0.060 ^{- - FeSi}

~ S 0.050

~

;; 0.040

0

"

^0.030

..",

;;

0.020 0.010 0.000

0 200

EXPERIMENTAL

• ..

'. ^.. . ^{. -} ... ^{- .. -- -} . ^{- - -} . ^{- -. - - . - - - . -}

^-

^.

.- - -- ^- - ^{- ---}

^-

-- ^{- .}

- - ^- - ^{- -- - - - ---}

400 600 800 1000 1200 1400 1600 1800 2000 Shear rate (S·l)

Figure 6.24 Effect of shear rate on viscosity for 270D ferrosilicon-magnetite #1 mixtures (sp.gr.2.7)

0.120 , . . - - - -- - - -_ _ _ _ _ _ --, - - 1:1

0.100 -.- 2:1

--

^..",

~. 0.080 Z S

--

^{C 0.060}

. ;;

o ~ 0.040

;;

0.020 0.000

- 1:2 - - FeSi

o

200

,

•

, , ,

..

^"-

_... ^... _...

...

- ^... - - --

-

.. ---

^-

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^.

^---

^{. -}

^.

400 600 800 1000 1200 1400 1600 1800 2000 Shear rate (S'l)

Figure 6.25 Effect of shear rate on viscosity for 270D ferrosilicon-magnetite #1 mixtures (sp.gr.2.8)

CHAPTER 6 EXPERIMENTAL

The graphs above were plotted together with viscosity values of pure ferrosilicon suspensions at the same specific gravity, in order to illustrate the increase in viscosity caused by the presence of magnetite. All the graphs show that there is an increase in the viscosity of suspensions due to the magnetite media. The graphs also show, for a constant specific gravity, an increase in viscosity as the amount of magnetite is increased. At a specific gravity of 2.2 the behaviour of the ferrosilicon-magnetite mixtures is close to Newtonian. At a specific gravity of 2.3, the behaviour of the mixtures begins to resemble that of a pseudo-plastic fluid, especially at higher magnetite content. From a specific gravity of 2.4 and above, the presence of a yield stress becomes prominent. As the amount of magnetite is increased, the yield stress value also increases. At these higher specific gravities, and higher magnetite ratios, the suspensions behave as Bingham plastic fluids. These results are similar to those found by Collins et al. (1983) and Lilge et al.

(l958). This illustrates the large increase in viscosity caused by the use of magnetite with ferrosilicon, particularly at high specific gravities.